Kinetics of austenite formation during continuous heating in as-cast and as-annealed conditions in a low carbon steel

Dilatometric analysis was used to study the effect of the initial microstructure on the kinetics of austenite formation during continuous heating in low-carbon steel. Two initial microstructures were analyzed: 1) as-cast condition, composed of columnar grains of Widmast & auml;tten ferrite and pearlite, and 2) as-annealed condition, formed of equiaxed grains of ferrite and pearlite. For both initial conditions, it was observed that austenite formation occurs in two stages: the first stage involves the decomposition of pearlite into austenite, and the second stage involves the transformation of ferrite into austenite. In the as-cast condition, a delay in austenite formation was observed, and a decrease in the transformation extent was associated with a higher transformation rate. The apparent activation energies for each stage of austenite formation were calculated using the Kissinger method, with the peak temperature at the point of maximum transformation rate as the criterion. The calculated energies were too high to be associated with a diffusive transformation mechanism, especially under as-annealed condition, so an analytical kinetic model was proposed to evaluate the behavior of the kinetic parameters. Additionally, the phases formed during intercritical annealing were analyzed to evidence the austenite formation and possible segregation of alloying elements. The analysis shows that elemental variations are negligible, so the difference lies mainly in the morphology of the initial condition.